The present invention relates to reagent for removing protective groups attached to amino acids during peptide synthesis.
In peptide synthesis, it is important to remove or cleave protective groups attached to amino acid at the final stage thereof and this removing reaction exerts a great influence on the result of such peptide synthesis. Hitherto, hydrogen fluoride has generally been used as reagent for removing protective groups, i.e., departing reagent.
However, it is required to strictly select containers and implements for handling hydrogen fluoride since it has high corrosive properties. For instance, a container or the like made of glass cannot be used and it has been obliged to employ those of fluoroplastics. Besides, hydrogen fluoride is harmful to human and, therefore, cautions for handling the same are necessary. Moreover, the use of hydrogen fluoride as the reagent for removing protective groups leads to the increase in by-products due to side reactions.
Under such circumstances, we, the inventors of this invention have conducted various studies to develop reagent for removing protective groups in place of hydrogen fluoride currently employed. As a result of the studies, we found out reagent comprising a combination of hard acid and soft base on the basis of the principle of hard-soft acid-base, the detail of which is disclosed in YAKUGAKU ZASSHI (J. Pharm. Soc. Japan), 1983, 103, 805-818. The method disclosed in this article comprises employing the combination of trifluoromethanesulfonic acid (CF.sub.3 SO.sub.3 H) as the hard acid with thioanisole as the soft base and carrying out the removal of protective groups under acidic condition. As trifluoromethanesulfonic acid used in this reaction is less harmful to humans compared with hydrogen fluoride, we may use cheap equipment. As side reaction is low possibility, we can get peptide by high yield. However, trifluoromethanesulfonic acid is fumy and very difficult to handle. Therefore, the reagent improved has been wanted.